Optical signal processing systems are used to manipulate the characteristics of optical signals, or light beams, to enable information or signals to be processed. For example, the direction, intensity, polarization, phase, or a combination of these characteristics of a light beam may be manipulated by appropriate equipment so that the manipulated characteristics impart the desired processing of the optical signal. For example, optical signal processing systems can be advantageously used for controlling phased array radars. In such a system, differentially time-delayed optical signals can be generated to establish selected time delays between individual optical signal beams, and the optical signals can then be converted to electrical signals to drive the antenna array with the desired sequential actuation of the transmitting antenna elements.
An essential component in most optical processing systems, such as phased array antenna controllers, is an efficient light switch. A key characteristic of a light switch used in a signal processing system, in which the processed light must commonly pass through many switches, is the amount of optical loss or attenuation that the light beam experiences in passing through the switch. A commonly used light switch is a lithium niobate based switch that provides relatively fast switching times but typically also has about 3 dB light loss per switch. Thus, for example, if an optical signal passed through only seven switches in a signal processing system, it would lose 21 dB, that is the light amplitude of the output signal drops to 1/128th of the input amplitude.
One efficient, low loss, light switch includes a beam splitter and a liquid crystal array to selectively control the polarization of light beams entering the beams splitter. One example of switching using polarizing beam splitters (PBS) and liquid crystal arrays to selectively control the polarization of light entering the PBSs is disclosed in U.S. Pat. No. 5,117,239 of N. Riza, issued May 26 1992 and which is assigned to the assignee of the present application and incorporated herein by reference. For example, separate spatial light modulators (SLMs) comprising arrays of liquid crystals can be paired with sets of paired polarizing beam splitters in optical time delay units such that light passing through the unit passes along either a direct path or a delay path dependent on the polarization of the light.
Additionally, polarization-independent types of beamsplitter switches have been suggested that enable a non-polarized light beam to be selectively directed along a predetermined path. One example of such a polarization-independent switch is described by Wagner and Cheng in "Electrically Controlled Optical Switch For Multimode Fiber Applications," Applied Optics, Vol. 19, No. 17, September 1980, pp 2921-2925. In optical signal processing systems, use of polarization independent switches can be advantageous as the there is no reduction in light beam intensity (as may occur if a polarizer is used to polarize light to be used in a polarization-dependent system) and connections between modules of the signal processing system can be made with optical fibers that do not require polarization-maintaining fibers.
It is desirable that switching units in optical signal systems have relatively low attenuation, be of compact size, rugged, readily fabricated and adapted to processing a large number of separate signal light beams as would be necessary for operation of a phased array antenna system. It is further desirable, from the standpoint of manufacturing ease and efficiency, that each optical switch comprise as few optical devices, such as beamsplitters, as practicable. Additionally, it is desirable to have a switching unit that can readily be fabricated to produce a small time delay.
It is accordingly an object of this invention to provide a polarization independent optical switching unit having relatively low light attenuation and that is readily adapted to use in a cascade of other optical processing devices.
It is a further object of this invention to provide a polarization independent optical switching unit that is compact and rugged and that can be readily fabricated in a cascade of similar devices.
It is a still further object of this invention to provide an arrangement for a polarization independent optical switching unit that is readily adapted to generating small time delays between respective optical signal beams.